Cerebral oximetry

Alternative methods for measuring cerebral blood flow, such as transcranial Doppler and radiological perfusion studies, are often operator-dependent and resource-intensive.

Coined in 1942 by physiologist Glen Milliken, the term “oximetry” refers to the measurement of haemoglobin oxygen saturation in blood or tissue

Light in the near-infrared spectrum penetrates biological tissues, including the skull and brain, and exhibits distinct absorption patterns in chromophores such as oxyhaemoglobin, deoxyhaemoglobin, and cytochrome c oxidase

If light attenuation between a source and detector is solely due to absorption by chromophores, it follows the Beer-Lambert law

Wavelengths typically used range from 700 to 870 nm, where the absorption spectra of haemoglobin (Hb) and oxyhemoglobin are maximally separated and overlap with H2O is minimal

The most common modification in commercial devices is multidistance (or spatially resolved) spectroscopy. This technique is based on the principle that the depth of tissue investigated is directly proportional to the distance between the light emitter and light detector. Increasing this distance can enhance the depth of tissue sampled, allowing for the measurement of oxygenation in the intracranial compartment.

Unlike pulse oximetry, which measures arterial blood oxygenation, cerebral oximetry assesses the entire returned signal, so pulsatility of tissue components is not required. It evaluates all haemoglobin in the reflectance arc (including those within the arterial, venous, and capillary compartments), resulting in a value for ScO2 that is biased toward the larger venous haemoglobin mass, or “venous-weighted.”

In healthy adults, the normal cerebral oxygen extraction ratio ranges from 20% to 40%, with a commonly cited “normal value” of ScO2 between 60% and 80%. Due to significant variation in baseline values, cerebral oximetry is best used as a trend monitor

There is no consensus on what decrement in cerebral oxygen saturation from baseline signifies irreversible injury.
a widely used criterion for defining “desaturation” is a reduction of >20% from baseline or an absolute value of <50%

Low ScO2 values may indicate inadequate oxygen delivery (e.g., cerebral hypoperfusion) or increased tissue oxygen extraction due to high metabolic demand (e.g., seizures).
Conversely, high ScO2 values may suggest cerebral hyperperfusion or metabolic suppression

Intravascular dyes, such as indocyanine green (with a characteristic absorption peak around 805 nm) and methylene blue (with a peak around 668 nm), may also affect readings.